The long term aim of this project is to g enetically study how the expression of genes can be influenced by their position in the genome. The initial studies will focus in detail on the genetic interactions of 4 SIR genes which control the position dependent expression of the a and @ (alpha) mating type genes in Saccharomyces cerevisiae. The initial experiments will fuse classical genetics with gene splicing techniques to characterize these interactions and to develop ways of studying the interactions on a biochemical level. In conjunction with studying this specific set of genes, I will initiate an exploration of the genetic properties of the genome as a whole, including studies of general position effects. As a first step, I propose to identify large recombinant clones corresponding to each segment of the entire yeast genome. This project is now feasible due to a new method for in vivo plasmid transfer described in this proposal. Two of the medically significant aspects of this proposal are noted. (1) The position of genes relative to surrounding sequences affects a genes expression in several cases: in humans in the disease hereditary persistanence of fetal hemoglobin and in mice where mammary tumor viruses are expressed or not depending upon their position in the genome. In neither case is the molecular basis understood. An analogous phenomenon in Saccharomyces, interesting in its own right, will give us information on possible molecular mechanisms. (2) The technique to be used for genome cloning will be developed for general utility so that it can be used to create genetic maps of other fungi including pathogens by taking advantage of S. cerevisiae's advanced genetics.